优化 Vercel Fluid CPU：API加CDN缓存、缩小middleware范围、跳过公共API的Supabase身份转发

- 7条GET路由加 s-maxage + stale-while-revalidate，fetch 改为 next revalidate
- middleware matcher 从全匹配缩小到9条需auth的路径，移除 isStaticAsset
- 8条公共API路由加 includeSupabaseIdentity: false
- backend-auth getSession 为空时跳过 getUser
- subscription-help 加 I18nProvider，移除 force-dynamic → 静态预渲染
- next.config.mjs 加静态资源 immutable 缓存头

Tested: npx tsc --noEmit 通过，npm run build 通过

frontend/app/subscription-help/page.tsx

@@ -1,7 +1,6 @@
 import type { Metadata } from "next";
 import { SubscriptionHelpClient } from "./SubscriptionHelpClient";
-
-export const dynamic = "force-dynamic";
+import { I18nProvider } from "@/hooks/useI18n";
 
 export const metadata: Metadata = {
   title: "PolyWeather | Subscription Help",
@@ -9,5 +8,9 @@ export const metadata: Metadata = {
 };
 
 export default function SubscriptionHelpPage() {
-  return <SubscriptionHelpClient />;
+  return (
+    <I18nProvider>
+      <SubscriptionHelpClient />
+    </I18nProvider>
+  );
 }